Laser machining apparatuses are extensively used in conjuction with a numerical control apparatus, because a workpiece can be machined thereby to form a complex shape at a high speed. In a conventional laser machining apparatus, a direction in which a laser beam is moved must be reversed while temporarily making a speed of the laser beam relative to the workpiece zero when the workpiece is machined to form a corner portion or an acute angle edge portion. That is, when machining the corner portion, the moving speed of the laser beam is decelerated, stopped, and then accelerated.
FIG. 5 illustrates a workpiece 40 machined by the laser beam to form a groove 41. To form the groove as illustrated, the laser beam is moved relative to the workpiece 40 in the direction indicated by an arrow A. Before reaching a corner portion 42, the speed at which the laser beam is moved is decelerated, the beam is temporarily stopped at the corner portion 42, and the beam is then accelerated after passing the corner portion 42. In this manner, the speed of the laser beam relative to the workpiece 40 is reduced when the laser beam passes through the corner portion 42. Therefore, a defective melting occurs in the corner portion due to a storage of thermal energy and excessive heat absorption when the machining is performed under not only a CW beam condition but also a pulsed beam condition. Thus the cutting accuracy and the quality of the final product are greatly reduced.
FIG. 6 is an enlargement of the corner portion shown in FIG. 5, in which the corner portion of the groove machined by the laser beam is illustrated in detail. Portions B and C indicated by oblique lines alongside of the corner portion are melted down due to the lowering of the relative speed of the laser beam, whereby the machining accuracy is lowered. Further, the thermally influenced regions are extended to portions D and E, to thereby exert adverse influence upon the material of the workpiece and increase the surface roughness of the engraved surface.
As a solution, the machining of the corner portion can be controlled in such a manner that a pulse duty ratio is lowered in accordance with to the lowering of the speed at which the laser beam is moved, to thereby reduce the laser output. This method is not satisfactory, however, in that a dross-free machining cannot be accomplished over an extensive region.